Recently, I read that someone's 额头上有一个伤痕 。 I knew that the sentence literally meant that there was a scar. But then I thought "Why didn't they say "伤疤” or “疤痕”。 All three of these translate to scar in english, is there any simple way to divide them in Chinese?
Often "疤痕" is reserved for "physical scar", while "傷疤" and "傷痕" can mean more; they can even mean a mental scar.
Therefore, being a 疤痕 implies being a 傷疤 (resp. 傷痕); being a 傷疤 (resp. 傷痕) not necessarily implies being a 疤痕.
Nevertheless, please bear in mind that these distinctions are subtle and not dogmatic. :)
There is a little difference between 伤痕 and 伤疤, but realy difficult to translate to english for both usually means scar.
伤疤 refers to the thing that cicatrizing after wounded, while 伤痕 is the scar left while the wound has been cured, and 疤痕 means the scar left after cicatrizing. Of course, both can be translated to scar, and can mean physical or mental scar. In fact, 疤痕 also can be used as mental scar left, like 心上的疤痕难以消除, but it's too formal and awkwardly（别扭）.
While all three can refer to a scar, 伤痕 is the only term that can also refer to a relatively fresh wound.
Example: "受害人躺在地上，身上滿布伤痕。" (The victim lay on the ground, his body covered with injury marks.)
In this sentence, the victim is possibly still bleeding, and hurting from the injuries, whereas a person who has 疤痕 or 傷疤 is no longer bleeding or hurting from those wounds.